It is indispensable to use a herbicide for protecting, and increasing the yields of, useful crops such as rice, wheat, barley, corn, soybean, cotton, beet and the like. In recent years in particular, there are desired selective herbicides which cause no phytotoxicity on the useful crops but selectively control weeds alone by foliar treatment of the crops and the weeds at the same time in a cultivated field where the useful crops and weeds are concurrently present.
During the planting time of corn, etc., triazine-based herbicides such as atrazine and acid anilide-based herbicides such as alachlor and metolachlor have been conventionally used as soil treating chemical. However, these herbicides require high dosage and are therefore causing environmental problems such as pollution of ground water.
In recent years, further, "non-tilled cropping" intended for soil conservation is promoted. "Non-tilled cropping" is a method of cultivating crops without plowing, and is against usual tilled cropping. In tilled cropping, fertile surface soil which is tilled (plowed) is washed away by rain, etc., which is not only a great farming problem but also a problem which leads to a kind of desertification. On the other hand, in non-tilled cropping, there is no problem of surface soil being washed away. However, without tilling, soil is hardened so that a chemical is scarcely infiltrated into the soil, and the effect of the chemical on soil treatment decreases. In the non-tilled cropping, therefore, there are demanded herbicides which have high herbicidal efficacy at a low dosage in soil treatment and which can be used as a single chemical for foliar treatment as well.
International Laid-open Patent Publication No. WO93/18031 discloses herbicidally active pyrazole derivatives having a thiochroman ring, represented by the following formula, ##STR1##
wherein R.sup.1 is a C.sub.1 -C.sub.6 alkyl group. Explanations of the other symbols are omitted.
Typical compound (A) of the above International Laid-open Patent Publication (Compound No. 66 in the Publication) has the following structural formula. ##STR2##
The above compound (A) shows excellent selectively between crops and weeds without damaging cultivated crops when used for the treatment of corn, wheat, barley, etc., at the stage of 1.about.2 leaves. However, when the foliar treatment is carried out at the stage of 3.about.4 leaves, the safety to the crops is not fully satisfactory.